A W-CDMA (Wideband-CDMA) system is a kind of CDMA (Code Division Multiple Access) , which is one of digital radio communication access systems. This W-CDMA standard specifies a DSCH as a downlink channel shared by a plurality of communication terminals.
This DSCH is a channel for transmitting only data with data allocated in predetermined transmission units (e.g., frame units) to a plurality of terminals, and is a channel for data communications. Therefore, the DSCH is expected to be used for downlink high-speed packet data transmission.
A communication terminal using a DSCH establishes a separate downlink (DPCH:Dedicated Physical CHannel) and carries out path search and channel estimation using known signals (e.g., pilot signals) included in the DSCH signal. Or path search and channel estimation are carried out using known signals of P-CPICH (Primary-Common PIlot CHannel) which is common to various communication terminals. This allows reliable demodulation of a DSCH signal.
Furthermore, HSDPA is available as one of communication methods for transmitting packet data at high speed using a downlink channel shared by a plurality of communication terminals. HSDPA is a communication method capable of improving average throughput by changing channel CODEC, spreading factor, multiplexing number or (multi-value) modulation according to the channel condition and changing a transmission rate to thereby improve average throughput.
On the other hand, in a cellular communication, a communication terminal apparatus moves and may switch between base station apparatuses with which it communicates.
For example, when handover takes place between W-CDMA systems or between W-CDMA and GSM (Global System for Mobile Communications) systems using carriers with different frequencies, in order to receive signals at different frequencies and switch between base stations, the communication terminal apparatus needs to measure necessary signal levels and receive necessary control information.
Therefore, in the case of W-CDMA, the base station apparatus sends a signal in a compressed mode and the communication terminal monitors information from the cell to which the own station belongs and information from a base station apparatus in an adjacent cell.
In a compressed mode, a spreading factor of information data with respect to a frame is temporarily reduced, frames are sent with a certain time interval with a reduced spreading factor created between frames and the communication terminal monitors information from the cell to which the own station belongs and a base station apparatus in an adjacent cell using this idle time.
That is, the base station apparatus creates a time during which no data is sent within a range that does not affect the communication and the communication terminal apparatus monitors information of the cell to which the own station belongs and a base station apparatus in an adjacent cell during this time.
When both a packet data transmission and compressed mode using shared channels such as the above-described DSCH and HSDPA are applied, the base station apparatus may send packet data to the communication terminal apparatus while the communication terminal apparatus is conducting carrier sensing with frequencies not used for communications in the compressed mode.
For example, when searching for another base station, the communication terminal apparatus may receive a signal with a frequency other than that used for reception of packet data, and therefore the communication terminal apparatus cannot receive packet data sent from the base station during this time. Furthermore, this packet data becomes an interference component to the other communication terminal apparatus. As a result, throughput of the overall system decreases.
Furthermore, when the circuit is adapted so as to be able to receive signals with a plurality of frequencies to receive packet data during carrier sensing, the problem is that this increases the circuit scale of the communication terminal apparatus.